The present invention relates to a press and a method for molding thermoplastics, thermosetting, and polyurethane polymeric articles.
In the present description, the term "molding" refers to both known methods of injection and injection-compression of fluid polymeric material using thermostat-fitted shell molds.
As is known, the current molding technology employs hydraulic presses with a fixed base and a movable plate accommodating therebetween the mold which receives the closing pressure from the movable plate which is subject to the action of one or more fluidodynamic jacks capable of exerting considerable stresses, for example comprised between 2500 and 3500 tons.
The molding process essentially consists of the following operative steps:
(a) closure and locking of the mold by clamping the same between the base and the movable plate
(b) advancement of the carriage bearing the injection machine
(c) injection
(d) return of the carriage
(e) unlocking
(f) waiting
(g) translatory motion and opening of the mold.
During the steps (d) to (g), cooling of the molded article occurs, thereafter the mold is opened and the article is extracted. Cooling is performed while maintaining on the mold a reduced closing pressure required for the correct execution of said operative step.
The cooling time, which substantially lasts from the end of the injection step to the opening of the mold, is considerable, generally comprised between two and three times the time required by the injection step, and is a dead time which considerably affects plant productivity and therefore the cost of the finished product.
If the articles to be molded are small, one tries to contain the negative effect of the cooling time by using multiple molds so that the dead cooling time is divided among the several articles obtained with a single molding.
This manner of molding--which besides entails a considerable increase in the cost of the mold--is not applicable in case of large articles, such as bumpers for automotive vehicles, dashboards and the like, since the molds, the presses and the injection machines would assume prohibitive costs and dimensions which are absolutely incompatible both with the movement requirements and with the economy of company management.